Low-alloy steels are commonly used for structural applications in shapes such as plates, bars, pilings, pipe and the like. Low-alloy steels are selected for such structural applications because they have good mechanical and physical properties, they are generally low in cost, and they have a high degree of versatility. The properties of such steels can be varied by either adjusting the alloying elements and/or altering the processing steps used to manufacture the steel into a final form. Typical final form applications for these types of steels include poles, ships, linepipe and other similar structural applications.
ASTM Designation A572/A572M is one standard for low-alloy steels containing niobium and vanadium. This specification sets an alloy content range, in weight percent, of up to 0.23% carbon, up to 1.65% manganese, up to 0.04% phosphorus, up to 0.05% sulfur, up to 0.40% silicon, up to 0.05% niobium, between 0.01 and 0.15% vanadium and up to 0.015% nitrogen with the balance iron and inevitable impurities. For grade 65 of this specification (generally of higher carbon and microalloy contents), the minimum yield strength is 65 ksi (450 MPa) and the minimum tensile strength is 80 ksi (550 MPa).
Subsequent to the development of the original ASTM A572 steel (a higher C-V grade), another alloy was developed containing lower amounts of carbon with vanadium and the addition of niobium (C-Nb-V). This steel permitted relaxation of the processing variables while still achieving the desired mechanical properties. One drawback associated with the C-Nb-V steel was the difficulty in achieving Charpy V-Notch toughness values. Pole manufacturers generally require a minimum longitudinal Charpy V-Notch (CVN) toughness of 15 foot-pounds (20.3 Joules) at -20.degree. F. (-29.degree. C.). To meet this requirement, reheating temperatures of the slab to be hot rolled were restricted to minimize austenite grain growth.
With a need to further improve the properties of the C-Nb-V alloy steels, a titanium-containing grade was developed (C-Nb-V-Ti). With the small addition of titanium, a fine dispersion of titanium nitride particles forms during cooling after solidification in a continuous caster. The particles restrict austenite grain growth during reheating and subsequent recrystallization steps. Consequently, the C-Nb-V-Ti grade is expected to be less sensitive to reheating temperatures, thereby providing more flexibility in the manufacturing process. For the titanium nitride technology to be particularly effective, the size of the titanium nitride particles should be small, this size being possible when the slab is produced by continuous casting.
Products produced from the C-Nb-V-Ti grade are generally air cooled after hot rolling. Although this grade exhibits superior levels of toughness than the C-Nb-V grade, meeting the ASTM A572 Grade 65 specifications for yield and tensile strengths requires precise processing controls to minimize off specification material. Such controls ultimately increase the overall costs of the product and manufacturing operation.
Consequently, a need has developed to improve the manufacturing process of these types of low-alloy steels in terms of productivity while still maintaining the minimal mechanical properties required, e.g., yield strength, tensile strength and CVN toughness. The present invention solves this need by providing a low-silicon steel containing controlled amounts of titanium, niobium, vanadium and carbon. The low-alloy steel is subjected to a controlled rolling and accelerated cooling sequence to produce a rolled product meeting minimal mechanical properties while providing for significant improvements in mill productivity.
In the prior art, the use of accelerated cooling of low-alloy steels has been disclosed. Japanese Publication No. 59-83722 to Kawasaki Steel discloses low-carbon steel plates produced by heating a slab comprising, among other alloying elements, silicon, niobium, boron and titanium. This steel is hot rolled and immediately subjected to forced cooling to a temperature lower than 500.degree. C. at a cooling rate of 2-30.degree. C. per second.
Japanese Publication No. 59-22528 to Sumitomo Metal Industries discloses another process of producing a rolled high-strength steel plate wherein the steel includes carbon, silicon, manganese, aluminum, vanadium, nitrogen and one of zirconium, a rare earth metal and calcium. The steel is continuously cast into a slab, hot rolled and accelerated cooled to below 250.degree. C. followed by coiling.
Japanese Publication No. 59-211528 to Nippon Steel Corporation discloses a low yield ratio for a steel containing carbon, 0.05 to 0.60 wt. % silicon, manganese, aluminum and at least one of chromium, nickel, molybdenum, vanadium, titanium, niobium, copper and calcium. The hot rolled steel is rapidly cooled with water and then tempered.
U.S. Pat. No. 5,514,227 to Bodnar et al. also teaches the accelerated cooling of a low-alloy steel. Bodnar et al. are concerned with a steel that has a minimum yield strength of 50 ksi and one that contains carbon, manganese, phosphorus, silicon, titanium, nitrogen and vanadium with the balance iron.
None of the prior art discussed above teaches the inventive method wherein a low-alloy steel containing controlled amounts of silicon, carbon, vanadium, titanium and niobium is subjected to a controlled rolling and accelerated cooling sequence to improve rolling productivity while maintaining mechanical properties. The product made from the process of the present invention as well as a composition for use in the process are also not disclosed in the prior art discussed above.